Mechanistic insights into CO2 conversion to CO using cyano manganese complexes.

Without the use of a photosensitizer, [Mn(bpy)(CO)3(CN)] (MnCN) can photochemically form [Mn(bpy)(CO)3]-, the active species for CO2 reduction. While cases of the axial X-ligand dissociating upon irradiation of fac-[M(N-N)(CO)3X] complexes (M = Mn or Re; N-N = bipyridine (bpy) ligand; X = halogen or pseudohalogen) are well documented, the axial cyanide ligand is retained when either [Mn(bpy)(CO)3(CN)] or [Mn(mesbpy)(CO)3(CN)], MnCN(mesbpy), are irradiated anaerobically. Infrared and UV-vis spectroscopies indicate the formation of [Mn(bpy)(CO)2(MeCN)(CN)] (s-MnCN) as the primary product during the irradiation of MnCN. An in-depth analysis of the photochemical mechanism for the formation of [Mn(bpy)(CO)3]- from MnCN is presented. MnCN(mesbpy) is too sterically hindered to undergo the same photochemical mechanism as MnCN. However, MnCN(mesbpy) is found to be electrocatalytically active for CO2 reduction to CO. Thus providing an interesting distinction between photochemical and electrochemical charge transfer.

Direct Synthesis of 1-Butanol with High Faradaic Efficiency from CO2 Utilizing Cascade Catalysis at a Ni-Enhanced (Cr2O3)3Ga2O3 Electrocatalyst.

Electrochemical transformation of CO2 into energy-dense liquid fuels provides a viable solution to challenges regarding climate change and nonrenewable resource dependence. Here, we report on the modification of a Cr-Ga oxide electrocatalyst through the introduction of nickel to generate a catalyst that generates 1-butanol at unprecedented faradaic efficiencies (ξ = 42%). This faradaic efficiency occurs at -1.48 V vs Ag/AgCl, with 1-butanol production commencing at an overpotential of 320 mV. At this potential, minor products include formate, methanol, acetic acid, acetone, and 3-hydroxybutanal. At -1.0 and -1.4 V, 3-hydroxybutanal becomes the primary product. This is in contrast to the nickel-free (Cr2O3)3(Ga2O3) system, where neither 3-hydroxybutanal nor 1-butanol was detected. Mechanistic studies show that formate is the initial CO2 reduction product and identify acetaldehyde as the key intermediate. Nickel is found responsible for the coupling and reduction of acetaldehyde to generate the higher molecular weight carbon products observed. To the best of our knowledge, this is the first electrocatalyst to generate 1-butanol with high faradaic efficiency.

Elucidating the mechanism of photochemical CO2 reduction to CO using a cyanide-bridged di-manganese complex.

The complex, [{[Mn(bpy)(CO)3]2}(µ-CN)]+ (Mn2CN+), has previously been shown to photochemically reduce CO2 to CO. The detailed mechanism behind its reactivity was not elucidated. Herein, the photoevolution of this reaction is studied in acetonitrile (MeCN) using IR and UV-vis spectroscopy. Samples were excited into the MnI → π* bpy metal-to-ligand charge transfer (MLCT) absorption band triggering CO loss, and rapid MeCN solvent ligation at the open coordination site. It is concluded that this process occurs selectively at the Mn axial ligation site that is trans to the C-end of the bridging cyanide. Upon further photolysis, the metal-metal bonded dimeric species, [(CO)3(bpy)Mn-Mn(bpy)(CO)3] (Mn-Mn) is observed to form under anaerobic conditions. The presence of this dimeric species coincides with the observation of CO production. When oxygen is present, CO2 photoreduction does not occur, which is attributed to the inability of Mn2CN+ to convert to the metal-metal bonded dimer. Photolysis experiments, where the Mn-Mn dimer is formed photochemically under argon first and then exposed to CO2, reveal that it is the radical species, [Mn(bpy)(CO)3˙] (Mn˙), that interacts with the CO2. Since the presence of Mn-Mn and light is required for CO production, [Mn(bpy)(CO)3˙] is proposed to be a photochemical reagent for the transformation of CO2 to CO.

Using Light and Electrons to Bend Carbon Dioxide: Developing and Understanding Catalysts for CO2 Conversion to Fuels and Feedstocks.

Our global society generates an unwieldy amount of CO2 per unit time. Therefore, the capture of this greenhouse gas must involve a diverse set of strategies. One solution to this problem is the conversion of CO2 into a more useful chemical species. Again, a multiplicity of syntheses and products will be necessary. No matter how elegant the chemistry is, commercial markets often have little use for a small set of compounds made in tremendous yield. Following this reasoning, the Bocarsly Research Group seeks to develop new electrochemical and photochemical processes that may be of utility in the conversion of CO2 to organic compounds. We focus on investigating proton-coupled charge transfer mechanisms that produce both C1 and carbon-carbon bonded products (C2+).In early work, we considered the reduction of CO2 to formate at electrocatalytic indium and tin electrodes. These studies demonstrated the key role of surface oxides in catalyzing the reduction of CO2. This work generated efficient systems for the formation of formate and paved the way to studies using non-copper, intermetallic electrocatalysts for the generation of C2+ species. Most notable is the efficient formation of oxalate at an oxidized Cr3Ga electrode. Oxalate has recently been suggested as a potential nonfossil, alternate organic feedstock.Separately, we have focused on the electrocatalytic effects of pyridine on the reduction of CO2 in aqueous electrolyte. These studies demonstrated that electrodes that normally yield a low hydrogen overpotential (Pd and Pt) show suppressed H2 evolution and strongly enhanced activity for CO2 reduction in the presence of pyridinium. Methanol was observed to form in high Faradaic yield at low overpotential using this system. The 6-electron, 6-proton reduction of CO2 in the presence of pyridinium was intriguing, and significant effort was placed on understanding the mechanism of this reaction both on metal electrodes and on semiconducting photocathodes. P-GaP electrodes were found to provide exceptional behavior for the formation of methanol using only light as the energy source.The pyridinium studies highlighted the role of protons in the overall reduction of CO2, stimulating our interest in the chemistry of MnBr(bpy)(CO)3 and related compounds. This complex was reported to electrochemically reduce CO2 to CO. We saw these reports as an opportunity to study the detailed nature of the proton-coupled electron transfer (PCET) mechanism associated with CO2 reduction. Our investigation of this system revealed the role of hydrogen-bonding in CO2 reduction and pointed the way for the construction of a photochemical process for CO generation using a [(bpy)(CO)3Mn(CN)Mn(bpy)(CO)3]+ photocatalyst.Based on our studies to date, it appears likely that heterogeneous systems can be assembled to convert CO2 into products that are "beyond C2 products." This may open up new practical chemistry in the area of fossil-based replacements for both synthesis and fuels. Systems with pragmatic efficiencies are close to reality. Electrochemical reactors using heterogeneous electrocatalysts show the stability and product selectivity needed to generate industrial opportunities. Continued growth of mechanistic understanding is expected to facilitate the chemical design of cogent systems for the taming of CO2.

Reduction-induced CO dissociation by a [Mn(bpy)(CO)4][SbF6] complex and its relevance in electrocatalytic CO2 reduction.

[Mn(bpy)(CO)3Br] is recognized as a benchmark electrocatalyst for CO2 reduction to CO, with the doubly reduced [Mn(bpy)(CO)3]- proposed to be the active species in the catalytic mechanism. The reaction of this intermediate with CO2 and two protons is expected to produce the tetracarbonyl cation, [Mn(bpy)(CO)4]+, thereby closing the catalytic cycle. However, this species has not been experimentally observed. In this study, [Mn(bpy)(CO)4][SbF6] (1) was directly synthesized and found to be an efficient electrocatalyst for the reduction of CO2 to CO in the presence of H2O. Complex 1 was characterized using X-ray crystallography as well as IR and UV-Vis spectroscopy. The redox activity of 1 was determined using cyclic voltammetry and compared with that of benchmark manganese complexes, e.g., [Mn(bpy)(CO)3Br] (2) and [Mn(bpy)(CO)3(MeCN)][PF6] (3). Infrared spectroscopic analyses indicated that CO dissociation occurs after a single-electron reduction of complex 1, producing a [Mn(bpy)(CO)3(MeCN)]+ species. Complex 1 was experimentally verified as both a precatalyst and an on-cycle intermediate in homogeneous Mn-based electrocatalytic CO2 reduction.

Elucidating the origins of enhanced CO2 reduction in manganese electrocatalysts bearing pendant hydrogen-bond donors.

Complexes of the general form [Mn(X)(CO)3bpy] (X = a variety of monodentate ligands, bpy = 2,2'-bipyridine) have been reported to act as electrocatalysts for the reduction of CO2 to CO. In this work, a series of phenol and anisole substituted bipyridine ligands were synthesized and ligated to a manganese metal center in order to probe for an intramolecular hydrogen-bonding interaction in the transition state of CO2 reduction. Ligands without the ability to intramolecularly hydrogen bond displayed decreased catalytic current density compared to those with the ability to hydrogen bond with CO2. Electrocatalysis was studied by performing voltammetric and bulk electrolysis experiments under argon or CO2 environments. Measurements of catalytic rates using hydrogen vs. deuterium for the intramolecular H/D-bonding step show that there is an isotope effect associated with the catalysis. The data presented herein suggest a mechanism involving two subsequent equilibrium isotope effects in combination with a primary kinetic isotope effect.

Mechanistic insights into C2 and C3 product generation using Ni3Al and Ni3Ga electrocatalysts for CO2 reduction.

Thin films of Ni3Al and Ni3Ga on carbon solid supports have been shown to generate multi-carbon products in electrochemical CO2 reduction, an activity profile that, until recently, was ascribed exclusively to Cu-based catalysts. This catalytic behavior has introduced questions regarding the role of each metal, as well as other system components, during CO2 reduction. Here, the significance of electrode structure and solid support choice in determining higher- versus lower-order reduction products is explored, and the commonly invoked Fischer-Tropsch-type mechanism of CO2 reduction to multi-carbon products is indirectly probed. Electrochemical studies of both intermetallic and non-mixed Ni-Group 13 catalyst films suggest that intermetallic character is required to achieve C2 and C3 products irrespective of carbon support choice, negating the possibility of separate metal sites performing distinct yet complementary roles in CO2 reduction. Furthermore, Ni3Al and Ni3Ga were shown to be incapable of generating higher-order reduction products in D2O, suggesting a departure from accepted mechanisms for CO2 reduction on Cu. Additional routes to multi-carbon products may therefore be accessible when developing intermetallic catalysts for CO2 electroreduction.

A cyanide-bridged di-manganese carbonyl complex that photochemically reduces CO2 to CO.

Manganese(i) tricarbonyl complexes such as [Mn(bpy)(CO)3L] (L = Br, or CN) are known to be electrocatalysts for CO2 reduction to CO. However, due to their rapid photodegradation under UV and visible light, these monomeric manganese complexes have not been considered as photocatalysts for CO2 reduction without the use of a photosensitizer. In this paper, we report a cyanide-bridged di-manganese complex, {[Mn(bpy)(CO)3]2(µ-CN)}ClO4, which is both electrocatalytic and photochemically active for CO2 reduction to CO. Compared to the [Mn(bpy)(CO)3CN] electrocatalyst, our CN-bridged binuclear complex is a more efficient electrocatalyst for CO2 reduction using H2O as a proton source. In addition, we report a photochemical CO2 reduction to CO using the dimanganese complex under 395 nm irradiation.

Single-Crystal Growth and Characterization of the Chalcopyrite Semiconductor CuInTe2 for Photoelectrochemical Solar Fuel Production.

Transition-metal chalcogenides are a promising family of materials for applications as photocathodes in photoelectrochemical (PEC) H2 generation. A long-standing challenge for chalcopyrite semiconductors is characterizing their electronic structure, both experimentally and theoretically, because of their relatively high-energy band gaps and spin-orbit coupling (SOC), respectively. In this work, we present single crystals of CuInTe2, whose relatively small optically measured band gap of 0.9 ± 0.03 eV enables electronic structure characterization by angle-resolved photoelectron spectroscopy (ARPES) in conjunction with first-principles calculations incorporating SOC. ARPES measurements reveal bands that are steeply dispersed in energy with a band velocity of 2.5-5.4 × 105 m/s, almost 50% of the extremely conductive material graphene. Additionally, CuInTe2 single crystals are fabricated into electrodes to experimentally determine the valence band edge energy and confirm the thermodynamic suitability of CuInTe2 for water redox chemistry. The electronic structure characterization and band edge position presented in this work provide kinetic and thermodynamic factors that support CuInTe2 as a strong candidate for water reduction.

Design of a Catalytic Active Site for Electrochemical CO2 Reduction with Mn(I)-Tricarbonyl Species.

The design, synthesis, and assessment of a new manganese-centered catalyst for the electrochemical reduction of CO2 is described. The reported species, MnBr(6-(2-hydroxyphenol)-2,2'-bipyridine)(CO)3, includes a ligand framework with a phenolic proton in close proximity to the CO2 binding site, which allows for facile proton-assisted C-O bond cleavage. As a result of this modification, seven times the electrocatalytic current enhancement is observed compared to MnBr(2,2'-bipyridine)(CO)3. Moreover, reduction is possible at only 440 mV of overpotential. Theoretical computations suggest that the entropic contribution to the activation free energy is partially responsible for the increased catalytic activity. Experimental work, including voltammetry and product quantification from controlled potential electrolysis, suggests a key mechanistic role for the phenolic proton in the conversion of CO2 to CO.

Exploring the effect of axial ligand substitution (X = Br, NCS, CN) on the photodecomposition and electrochemical activity of [MnX(N-C)(CO)3] complexes.

The synthesis, electrochemical activity, and relative photodecomposition rate is reported for four new Mn(i) N-heterocyclic carbene complexes: [MnX(N-ethyl-N'-2-pyridylimidazol-2-ylidine)(CO)3] (X = Br, NCS, CN) and [MnCN(N-ethyl-N'-2-pyridylbenzimidazol-2-ylidine)(CO)3]. All compounds display an electrocatalytic current enhancement under CO2 at the potential of the first reduction, which ranges from -1.53 V to -1.96 V versus the saturated calomel electrode. Catalytic CO production is observed for all species during four-hour preparative-scale electrolysis, but substantial H2 is detected in compounds where X is not Br. All species eventually decompose under both 350 nm and 420 nm light, but cyanide substituted complexes (X = CN) last significantly longer (up to 5×) under 420 nm light as a result of a blue-shifted MLCT band.

Anodized indium metal electrodes for enhanced carbon dioxide reduction in aqueous electrolyte.

The interactions of CO2 with indium metal electrodes have been characterized for electrochemical formate production. The electrode oxidation state, morphology, and voltammetric behaviors were systematically probed. It was found that an anodized indium electrode stabilized formate production over time compared to etched indium electrodes and indium electrodes bearing a native oxide in applied potential range of -1.4 to -1.8 V vs SCE. In addition, it was observed that formate is the major product at unprecedentedly low overpotentials at the anodized surface. A surface hydroxide species was observed suggesting a mechanism of formate production that involves insertion of CO2 at the indium interface to form an electroactive surface bicarbonate species.

NHC-containing manganese(I) electrocatalysts for the two-electron reduction of CO2.

The synthesis and characterization of the first catalytic manganese N-heterocyclic carbene complexes are reported: MnBr(N-methyl-N'-2-pyridylbenzimidazol-2-ylidine)(CO)3 and MnBr(N-methyl-N'-2-pyridylimidazol-2-ylidine)(CO)3. Both new species mediate the reduction of CO2 to CO following two-electron reduction of the Mn(I) center, as observed with preparative scale electrolysis and verified with (13)CO2. The two-electron reduction of these species occurs at a single potential, rather than in two sequential steps separated by hundreds of millivolts, as is the case for previously reported MnBr(2,2'-bipyridine)(CO)3. Catalytic current enhancement is observed at voltages similar to MnBr(2,2'-bipyridine)(CO)3.

p-type CuRhO2 as a self-healing photoelectrode for water reduction under visible light.

Polycrystalline CuRhO2 is investigated as a photocathode for the splitting of water under visible irradiation. The band edge positions of this material straddle the water oxidation and reduction redox potentials. Thus, photogenerated conduction band electrons are sufficiently energetic to reduce water, while the associated valence band holes are energetically able to oxidize water to O2. Under visible illumination, H2 production is observed with ~0.2 V underpotential in an air-saturated solution. In contrast, H2 production in an Ar-saturated solution was found to be unstable. This instability is associated with the reduction of the semiconductor forming Cu(s). However, in the presence of air or O2, bulk Cu(s) was not detected, implying that CuRhO2 is self-healing when air is present. This property allows for the stable formation of H2 with ca. 80% Faradaic efficiency.

Direct reduction of carbon dioxide to formate in high-gas-capacity ionic liquids at post-transition-metal electrodes.

As an approach to combat the increasing emissions of carbon dioxide in the last 50 years, the sequestration of carbon dioxide gas in ionic liquids has become an attractive research area. Ionic liquids can be made that possess incredibly high molar absorption and specificity characteristics for carbon dioxide. Their high carbon dioxide solubility and specificity combined with their high inherent electrical conductivity also creates an ideal medium for the electrochemical reduction of carbon dioxide. Herein, a lesser studied ionic liquid, 1-ethyl-3-methylimidazolium trifluoroacetate, was used as both an effective carbon dioxide capture material and subsequently as an electrochemical matrix with water for the direct reduction of carbon dioxide into formate at indium, tin, and lead electrodes in good yield (ca. 3 mg h(-1) cm(-2)).

Electrochemistry of aqueous pyridinium: exploration of a key aspect of electrocatalytic reduction of CO2 to methanol.

The mechanism by which pyridinium (pyrH(+)) is reduced at a Pt electrode is a matter of recent controversy. The quasireversible cyclic voltammetric wave observed at -0.58 V vs SCE at a Pt electrode was originally proposed to correspond to reduction of pyrH(+) to pyridinyl radical (pyrH(•)). This mechanistic explanation for the observed electrochemistry seems unlikely in light of recent quantum mechanical calculations that predict a very negative reduction potential (-1.37 V vs SCE) for the formation of pyrH(•). Several other mechanisms have been proposed to account for the discrepancy in calculated and observed reduction potentials, including surface adsorption of pyrH(•), reduction of pyrH(+) by two electrons rather than one, and reduction of the pyrH(+) proton to a surface hydride rather than a π-based radical product. This final mechanism, which can be described as inner-sphere reduction of pyrH(+) to form a surface hydride, is consistent with experimental observations.